Embodiments of the invention relate generally to tomographic imaging and, more particularly, to an apparatus and method of acquiring tomographic imaging data and increasing temporal resolution of a tomographic image.
Typically, in x-ray systems, such as a computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped or cone-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam of radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces an electrical signal indicative of the attenuated beam received by the detector element. The electrical signals are converted to digital signals and transmitted to a data processing system for analysis, which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam from a focal point. X-ray detectors typically include a collimator for collimating x-ray beams directed toward the detector, a scintillator adjacent to the collimator for converting x-rays to light energy, and photodiodes for receiving the light energy from the scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy and discharges the light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are digitized and then transmitted to the data processing system for image reconstruction. The x-ray detector extends typically over a circumferential angular range or fan angle of 60°.
CT imaging encompasses multiple configurations. For example, one configuration includes multi-slice or multi-detector CT imaging (MDCT), which may be employed for cardiac imaging. Such a system may be used to generate a cardiac image using imaging data that is obtained over a portion or phase of a cardiac cycle. Conventionally, the minimum projection angle of imaging data for image reconstruction is 180° of gantry rotation plus the x-ray detector fan angle. Thus, with a typical fan angle of 60°, the minimum projection angle or temporal aperture is 240° of projection data for image reconstruction, and projection data obtained over this “half-scan” or “short scan” range of coverage may be reconstructed using known reconstruction techniques. The amount of time taken to obtain the half-scan projection dataset together with the reconstruction algorithm, in this conventional example, defines the temporal resolution of the imaging system. In other words, the temporal resolution is defined as the time taken to obtain minimally adequate data for image reconstruction and the data actually used in the reconstruction. In this case, short scan data is obtained for 240° of gantry rotation with some type of weighting function, as is understood in the art.
As such, the range of angular coverage (or temporal aperture) and gantry rotational speed are primary factors that define temporal resolution in a MDCT scanner. In a typical single source MDCT scanner, temporal resolution is thus approximately 135 ms for a gantry rotational speed of 270 ms, and approximately 175 ms for a gantry rotational speed of 350 ms with a Parker weighting, as examples. In many imaging applications, such temporal resolution is adequate to provide images with acceptable motion artifacts.
Due to motion of the heart during the 240° of gantry rotation during which short scan data is obtained, however, the temporal resolution may be inadequate, and images reconstructed with short scan data can suffer from blurring, streaking, or other imaging artifacts. Thus, it is desirable to increase temporal resolution in cardiac imaging applications and in applications in general where imaging artifacts may occur due to object motion. In some applications, it would be desirable to increase the temporal resolution by a factor of up to 2, or even greater, in order to improve images and reduce or eliminate image artifacts.
Temporal resolution could be improved by increasing the gantry speed and thereby decreasing overall acquisition time. As such, artifacts may be reduced or eliminated because acquisition occurs over a smaller time period. Generally, however, weight of the gantry components and other forces acting on the gantry limit the speed at which the gantry can operate, and a reduction in the acquisition time typically includes more powerful x-ray tubes in order to achieve comparable image quality. As is known in the art, though, load on the gantry increases generally as a factor that is squared with respect to gantry rotational speed. Thus there are life, reliability, and performance considerations to take into account, and it is highly nontrivial to maintain stability and functionality of components on the gantry at increased gantry speeds.
Another technique to improve temporal resolution includes a two-tube/two-detector system. In such a system, two tubes operate simultaneously, thus decreasing overall acquisition time and increasing the temporal resolution as compared to a single source system. The cost, however, of two-tube/two-detector CT systems can be prohibitive. In addition, limited space on the gantry prevents the placement of two x-ray tubes and two full-FOV detectors. Thus, the second detector often covers only a fraction of the desired scan FOV. Further, a two-tube/two-detector CT system typically includes significantly more utility resources (i.e., coolant flow, electrical) when compared to a single tube system. Thus, imaging suites containing such systems sometimes need significant and costly upgrades to provide the additional utility supply. And, with an increased number of operational components, reliability of the overall system may be compromised because of the doubling in the number of primary components (i.e., tube, detector, and DAS). Thus, though such a system may improve temporal resolution, the increased temporal resolution comes at the cost of increased initial system expense and cost of ongoing operation, costly suite upgrades, and possibly a reduced system reliability when compared to a single source system.
Further, other imaging modalities such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) also suffer from blurring and other image artifacts due to cardiac or respiratory motions. Such blurring may be caused by inadequate data acquisition during a given acquisition, or may be caused by an inordinate amount of time that may be used in order to obtain tomographic imaging data having reduced blurring and image artifact characteristics.
Thus there is a need for a system and method that minimizes motion blurring in tomographic imaging in a cost-effective and overall efficient manner.